The present invention relates to a process for the preparation of 1-substituted 2,4-dinitrobenzenes by reacting 1-halogeno-2,4-dinitrobenzenes with mono-alkali metal salts of diols.
1-substituted 2,4-dinitrobenzenes are used in many different areas. Their synthesis is therefore of particular importance. 2-(2',4'-dinitro-phenoxy)-ethanol is, for example, a suitable starting material for the preparation of 2-(2',4'-diaminophenoxy)-ethanol and its salts, which are used in oxidation dye compositions as meta components (DE-A-2,758,735 and DE-A-2,737,138). 2-(2',4'-dinitrophenoxy)-ethanol is also an excellent plasticizer for cellulose acetate.
According to DE-A-2,758,735, the preparation of 2-(2',4'-dinitrophenoxy)-ethanol starts from an initial charge of mono-potassium glycolate in ethylene glycol, to which 1-chloro-2,4-dinitrobenzene is added. The mono-potassium glycolate is present in a 1.4-fold molar excess. However, this process produces 2-(2',4'-dinitrophenoxy)-ethanol only in a yield of 68.6%. From the fact that the melting point of the resulting product is given as 100-102.degree. C., while pure, crystalline 2-(2',4'-dinitrophenoxy)-ethanol has, according to German Patent Specification 479,831, a melting point of 111-112.degree. C., it can be concluded that the product from DE-A-2,758,735 also contains significant impurities.
J. Chem. Soc. 1921 (119), 2076-8 discloses a further process for the preparation of 2-(2',4'-dinitrophenoxy)-ethanol, in which a solution of 1-chloro-2,4-dinitrobenzene in ethylene glycol is the initial charge and a mixture of sodium hydroxide, water and ethylene glycol is metered into this initial charge. The mixture of sodium hydroxide, water and ethylene glycol is obtained by dissolving the solid sodium hydroxide in water and then adding the ethylene glycol. Using this process, a crude yield of 93% is achieved, in which the crude product is subsequently subjected to recrystallization from acetic acid for purification of the 2-(2',4'-dinitrophenoxy)-ethanol. In this reaction, the undesired by-product 1,2-bis-(2',4'-dinitrophenoxy)-ethane forms. Experiments are also described in which the mixture of sodium hydroxide, water and ethylene glycol is not prepared by adding the ethylene glycol to the aqueous sodium hydroxide solution, but, vice versa, by adding the aqueous sodium hydroxide solution to the ethylene glycol. It is stated that, as a result, the formation of the by-product 1,2-bis-(2',4'-dinitrophenoxy)-ethane can be avoided. However, a reworking of these experiments showed that even in this variant 1,2-bis-(2',4'-dinitrophenoxy)-ethane forms as 8% of the total yield.
U.S. Pat. No. 2,988,571 describes a process for the preparation of 2-(2',4'-dinitrophenoxy)-ethanol, in which the formation of the by-product 1,2-bis-(2',4'-dinitrophenoxy)-ethane and 1,2-bis-(2',4'-dinitrophenoxy)-ethanol is said to be repressed. For this purpose, 1-chloro-2,4-dinitrobenzene and solid, powdered, largely anhydrous sodium hydroxide, each in small portions, are simultaneously metered in to an initial charge of ethylene glycol at temperatures of 30-130.degree. C., preferably 85-90.degree. C. An important factor here is that fresh portions of the two substances are in each case only added again when the previously added portions have reacted completely. A decisive factor is also that the molar ratio of the ethylene glycol to the 1-chloro-2,4-dinitrobenzene is at least 3, and the molar ratio of the 1-chloro-2,4-dinitrobenzene to the sodium hydroxide is about 1.1-1.3. Then, water is added to the reaction system in an amount such that the 2-(2',4'-dinitrophenoxy)-ethanol is produced as a precipitate. In a preferred embodiment of the process, a large number of extremely small portions is added, such that a quasi continuous addition of the 1-chloro-2,4-dinitrobenzene and of the sodium hydroxide results. In this way, a yield of 86.5% of 2-(2',4'-dinitrophenoxy)-ethanol, based on the 1-chloro-2,4-dinitrobenzene used, is obtained. However, despite this the reaction product contains about 8% of the by-product 1,2-bis-(2',4'-dinitrophenoxy)-ethane (Test 1). If the molar ratio of the 1-chloro-2,4-dinitrobenzene to the sodium hydroxide is increased beyond the given range, the proportion of the by-product 1,2-bis-(2',4'-dinitrophenoxy)-ethane even increases to 23% (Test 2).
Particularly when 1-substituted 2,4-dinitrobenzenes such as 2-(2',4'-dinitrophenoxy)-ethanol, are used in the cosmetics sector, e.g., as precursor compounds in the field of hair cosmetics, the purity of the 1-substituted 2,4-dinitrobenzenes is an important factor: even slight contamination by foreign substances is problematical for medicinal reasons (e.g., danger of triggering allergies).
The object of the present invention is therefore to provide a process which permits the preparation of 1-substituted 2,4-dinitrobenzenes and in particular of 2-(2',4'-dinitrophenoxy)-ethanol not only with very good yields, but also with high purities.